I. Field of the Invention
The present invention is directed to metalworking, and more particularly to a process for hardening ferrous surfaces.
II. Description of the Prior Art
It has long been known to harden the surfaces of ferrous articles by annealing, nitriding or nitrocarburizing (also known as carbonitriding) those surfaces. Known hardening techniques involve the controlled heating of the article, or the controlled heating of the article while simultaneously exposing the surface of the article to either a molten salt bath, or an atmosphere, containing reactive sources of nitrogen and/or oxygen. The bath or atmosphere can also contain a source of carbon, in addition to either or both of nitrogen and oxygen.
Typically, such an atmosphere comprises an inert carrier such as nitrogen gas, while the reactive source of nitrogen is ammonia gas, and the reactive source of carbon is carbon monoxide, carbon dioxide, methene, ethene, propane or other hydrocarbon. Carbon monoxide and carbon dioxide are, of course, sources of oxygen as well. Cyanide can be used as a source of both carbon and nitrogen. Sulfur (for example, as sulfur dioxide) can also be included in the atmosphere. Hydrogen gas is often employed as an actuator to facilitate the reaction of the nitrogen, carburizing and sulfiding sources with the ferrous surface. The exposing step is carried out in either a circulating or convection oven and in some cases within a fluidized bed.
While these processes have yielded hardened surfaces on ferrous articles, their use has entailed some drawbacks. Single step processes provide only a single layer of hardened material on the article. For example, the layer provided by simultaneously oxidizing and nitriding or nitrocarburizing a ferrous surface is typically referred as an epsilon nitride or a white layer, although it of course contains oxygen as well. Plural step processes, which entail changing or adding additional material sources during the process, do result in a deposition of second new layer atop an existing layer. However, these plural step processes are not useful if an existing layer is destroyed by a subsequent layer-forming step, as may often be the case. In any event, prior processes have required a relatively long time to provide the desired hardened surfaces on ferrous articles. Additionally, prior processes have not yielded hardened surfaces having an optimal combination of resistance to wear, galling, case crushing and surface fatigue.